The mechanism of sleep is incompletely understood. Stimulation in the brainstem in a large region known as the reticular activating system causes arousal and low-voltage, fast EEG activity. Sleep was once thought to be caused by a reduced level of activity in the reticular activating system. However, substantial data, including the observations that anesthesia of the lower brainstem results in arousal and that stimulation in the medulla near the nucleus of the solitary tract can induce sleep, suggest that sleep is an active process. Investigators have tried to find a relationship between sleep mechanisms and brainstem networks in which particular neurotransmitters, including serotonin, norepinephrine, and acetylcholine, are used; manipulations of the levels of these transmitters in the brain can affect the sleep-wake cycle. However, a detailed neurochemical explanation of the neural mechanisms of sleep is not yet available.
Similarly, the purpose of sleep is still unclear. However, it must have a high value because so much of life is spent in sleep and because lack of sleep can be debilitating. Medically important disorders of the sleep-wake cycle include insomnia, bed-wetting, sleepwalking, sleep apnea, and narcolepsy.
•Fig. 10.6 Electroencephalographic (EEG) Abnormalities in Several Forms of Epilepsy. A, EEGtracingsduringthetonic(left) andclonic(right) phasesofatonic-clonic(grandmal)seizure.B, Spikeandwavecomponentsofanabsence(petitmal)seizure.C, EEGtracinginapersonwithtemporallobeepilepsy.D, EEGtracingofafocalseizure.(RedrawnfromEyzaguirreC,FidoneSJ.Physiology of the Nervous System. 2nded.St.Louis:Mosby;1975.)
TheEEGbecomesabnormalinavarietyofpathologicalcircumstances.Forexample,duringcoma,theEEGisdominatedbydeltaactivity.Brain death isdefinedbyamaintainedflatEEGwave.
Epilepsy commonlycauses,andcanbediagnosedby,specificEEGabnormalities.Therearemanyformsofepilepsy,andexamplesofEEGpatternsfromsomeofthesetypesofepilepsyareshownin
Figure10.6 .Epilepticseizurescanbeeitherpartialorgeneralized.
Oneformofpartialseizuresoriginatesinthemotorcortexandresultsinlocalizedcontractionsofcontralateralmuscles.Thecontractionsmaythenspreadtoothermuscles;suchspreadfollowsthesomatotopicsequenceofthemotorcortex(see ).ThisstereotypicalprogressioniscalledaJacksonian march. Complexpartialseizures(whichmayoccurinpsychomotor epilepsy)originateinthelimbicstructuresofthetemporallobeandresultinillusionsandsemipurposefulmotoractivity.Duringandbetweenfocalseizures,scalprecordingsmayrevealEEGspikes(see
Fig.
10.6C andD
Generalizedseizuresinvolvewideareasofthebrainandlossofconsciousness.Twomajortypesarepetit mal and
Although right-handedness represents a sensorimotor dominance of the left hemisphere and left-handedness represents a sensorimotor dominance of the right hemisphere, cerebral dominance is assigned to the hemisphere in which language is to communicate; in humans, the left hemisphere is the dominant hemisphere in more than 90% of both rightand left-handed people. This dominance grand mal seizures. Inpetitmalepilepsy,consciousnessislosttransiently(typicallyforlessthan15seconds),andtheEEGdisplaysspike and wave activity (see